1. 1 | Program Name or Ancillary Texteere.energy.gov Solar Energy Technologies Program Peer Review Improved Fullerenes for OPV Michael D Diener TDA Research 303 940 2314 May 26, 2010 PV

2. 2 | Solar Energy Technologies Programeere.energy.gov 2 Project start date: 8/8/2007 Project end date: 8/7/2009 Percent complete: 100% OPV are not sufficiently efficient; this project will increase the efficiency of organic photovoltaics. Total project funding –DOE share: $850,000 –Contractor share: $185,227 DOE Funding received in FY09: $379,490 DOE Funding for FY10: $0 Timeline Budget Barriers TDA Research, lead NREL, sub Partners Overview

3. 3 | Solar Energy Technologies Programeere.energy.gov Challenges, Barriers or Problems Though rapidly improving, the efficiency of organic photovoltaic (OPV) cells remains low Due to their extremely versatile and low-cost fabrication, a few percent additional increase in OPV efficiency will lead to their wide-spread adoption in a tremendous variety of power- generation applications. OPV Champion Device Efficiency by Year

4. 4 | Solar Energy Technologies Programeere.energy.gov 4 Objective: To increase the efficiency of OPV by increasing the open circuit voltage (V oc ) through the synthesis of new electron-rich fullerenes, used as acceptors in a variety of OPV architectures –V oc in OPV  (ionization potential of the polymer) – (electron affinity of the fullerene). Reduce the fullerene’s electron affinity, increase V oc. –Current V oc is ~0.5 V –2020 target efficiency is 12% (2008-2012 MYPP) 2010 champion device efficiency is 7.4% Need new materials that maintain low-cost manufacturing –2009 objectives were the de novo synthesis and characterization of electron-rich fullerene derivatives, followed by testing of the new fullerenes in OPV devices –New materials for OPV are created by synthetic organic chemistry. Good: incredibly large choice of materials; fine tailoring of properties Bad: de novo synthesis can be slow & costly Relevance

5. 5 | Solar Energy Technologies Programeere.energy.gov 5 Summarized Project Tasks –Optimize OPV performance from the materials developed in Phase I –Perform quantum chemical modeling of new synthetic targets –Synthesize the new electron-rich fullerene derivatives using the methodology developed in Phase I –Characterize the new fullerenes Electrochemistry UV-vis absorbance Solubility Stability –Test their performance in OPV Inverted bulk heterojunction (BHJ) cells with poly(3-hexylthiophene (P3HT) ITO/Mg x Zn 1-x O/P3HT:fullerene/(PEDOT:PSS or oxide/)Ag device geometry Approach Iterate

6. 6 | Solar Energy Technologies Programeere.energy.gov 6 Electron-rich elements tend to react directly with electron-poor fullerenes, without altering the electron affinity from that of PCBM Must ensure that the extra electron density is present in the lowest unoccupied molecular orbital (LUMO) of the resulting derivative Quantum chemistry calculations allow for downselection of targets –Example: C 60 C(CH 2 N(CH 2 ) 2 ) 2 Electron Affinity = 2.399 eV (vs. PCBM = 2.522 eV) MO52X/6-311++G(d,p) calculation The LUMO is on the fullerene: Approach

7. 7 | Solar Energy Technologies Programeere.energy.gov 7 NREL –$100,000 subcontract using a CRADA –Preparation and testing of the promising new fullerene derivatives in BJ OPV using their state-of-the-art facilities Collaborations

8. 8 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Three step synthesis of PCPZEA. The resulting isomer mixture is converted to pure (6,6)PCPZEA by stirring the purified isomer mixture under a sodium lamp for four hours. 1: NEt 3, CH 2 Cl 2, 0  C, 1h; H 2 O, MgSO 4, LC (silica); 32% yield. 2: CH 3 OH, 6h reflux; -CH 3 OH, +CH 2 Cl 2 ; H 2 O, MgSO 4, LC (silica); 20% yield. 3: NaOCH 3, pyridine, oDCB, 70  C, 16h, dark; -pyridine, -oDCB, LC (silica) 2x; 50% yield (consumed C 60 basis). Initial Synthetic Strategy New products compared to PCBM – similar solubility, similar morphology expected

9. 9 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results ITO/PEDOT:PSS/PCPZEA:P3HT/LiF/Al The new fullerenes do not work in normal devices with low work function metals ITO/PEDOT:PSS/(fullerene):P3HT/Ba/Al

10. 10 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results PCPZEA does work in inverted devices Tuning the work function of the TCO electrode greatly enhances efficiency ITO/Zn 1-x Mg x O/P3HT:PCPZEA/Ag devices cast from ODCB

11. 11 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Synthesis of PCSME Thermally unstable

12. 12 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Quantum Chemical Calculations for PCSME: Structure Determination Molecular mechanics conformational analysis: vary the dihedral angles, minimize the energy 10,000 optimizations: each of the 200 lowest energy structures was found about 50 times The six lowest energy conformers (of 200) all had the N pointing away from C 60 N in blue, O in red

13. 13 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Quantum Chemical Calculations: Electron Affinity Four lowest energy conformers + #7 (amine down) geometry optimized with M052X/6-31G #7 is 5 kcal/mol higher in energy than #1 Thermal energy at ambient temperature = 0.59 kcal/mol Not much #7 likely to be present Unless the crystal lattice energy imposes a higher energy conformation… Single point energy calculated at M052X/6-311++G(d,p) Rather little difference in electron affinity between PCBM and PCSME (either conformer) or TCSMe C 60 C(CH 2 N(CH 2 ) 2 ) 2 still looks good EA (eV) C 60 2.607 PCBM2.522 C 60 CH 2 2.523 PCSME conformer 12.486 PCSME conformer 72.488 TCSME conformer 12.522 C 60 C(CH 2 N(CH 3 ) 2 ) 2 2.399

14. 14 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Where Are the Electrons Going? Idea #1: The ester is stealing them Replace the ester with an alkyl chain Conformational analysis & structure optimization Electron Affinity is now 2.521 eV Same as PCBM Not the ester Idea #2: The phenyl ring is stealing them Replace the phenyl ring with a t-butyl group Conformational analysis & structure optimization Electron affinity is now 2.459 eV Halfway between PCBM and C 60 C(CH 2 N(CH 2 ) 2 ) 2 Yes, it’s the phenyl ring, combined with having an amine on both sides of the vertex carbon

15. 15 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Other Synthetic Targets with Amines Imidazoline Adduct Electron Affinity = 2.436 eV 3,6-diamine substituted cyclohexyl (A Diels-Alder adduct?) Electron Affinity = 2.354 eV

16. 16 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Silyl Adducts (CH 3 ) 2 SiC 60 Electron Affinity = 2.393 eV ((CH 3 ) 2 Si) 2 CC 60 Electron Affinity = 2.501 eV (Not useful) C 60 C(CH 3 ) 2 Electron Affinity = 2.497 Recent work from Japan shows SIMEF has ~0.1 eV lower electron affinity than PCBM (JACS 131, 16048), and OPV with phthalocyanine has PCE = 5.2%

17. 17 | Solar Energy Technologies Programeere.energy.gov Accomplishments / Progress / Results Other syntheses, other calculations, other devices not yet IP-protected Stability of electron-rich fullerene derivatives is clearly an issue: rearrangements and oxidations are frequent (and frustrating)

18. 18 | Solar Energy Technologies Programeere.energy.gov Budget Status and Potential for Expansion DOE $750,000 Phase II + $100,000 Phase I TDA $185,227: Equipment $60,283 + Labor –Project and budget are complete –Additional funding would allow us to pursue new derivatives Enhance the stability of the new derivatives through the introduction of bulky substituents and/or other chemical motifs Increased purity of the new derivatives –Only ~98% achieved routinely, impairing performance –Commercial electronic grade PCBM is 99.5%

19. 19 | Solar Energy Technologies Programeere.energy.gov Future Plans (FY 2011 and beyond) Pursue patent protection on the composition of matter of the new fullerenes, as well as the synthetic methodology Market the materials to OPV manufacturers Attempt to further enhance purity of the stable new fullerene derivatives

20. 20 | Solar Energy Technologies Programeere.energy.gov 20 OPV is swiftly advancing – efficiency has doubled in ~6 years and there is no sign of advancements slowing down –Expect to meet the 12% goal by ~2015 at this pace, ahead of the MYPP target of 2020 While cell construction can enhance efficiency, the big steps are taken with new materials Excellent progress has been made with low bandgap polymers to enhance currents, but little published work has appeared with new fullerenes to enhance V oc QC calculations prove that significant enhancements in performance are possible, but new derivatives must also have proper solubility and stability Summary